Wavelength division multiplexing (WDM) systems typically comprise multiple, separately modulated, optical carrier signals, each one being assigned to a different channel slot, or frequency, in the WDM signal. The sources for the carriers can be located at a single head-end in long-haul applications or remote from each other, with the channels typically being accumulated onto a single fiber by multiplexers, in metro area network applications, for example. Along the fiber transmission link, the carriers can be regenerated or collectively amplified typically by gain fiber, semiconductor optical amplifiers (SOA), or doped waveguide devices. At the link termination, the carriers are usually demodulated or separately routed to new links.
Spectral information is required to confirm the proper operation of WDM systems. Generally, the types of information required are different depending on the type of system. Long haul systems are many times concerned with information such as the spectral shape of the channels and optical noise level. Optical signal to noise ratio (OSNR) is a common metric that is required by these systems. The spectral information is used to control the amplifiers and possibly correct for any gain tilt. Networks that possibly include add-drop or cross-connect devices are typically interested in channel slot occupancy information, i.e., whether or not a carrier signal is present in a given channel slot. They also typically monitor whether or not specific carriers are located to their assigned channel slot frequency and/or whether or not they are operating at the correct power level.